1. The Field of the Invention
The present invention relates to the field of integrated optics including a semiconductor optical amplifier that reduces cross talk. More particularly, the present invention relates to a semiconductor optical amplifier that reduces or eliminates cross talk while using the gain medium of a broad area semiconductor laser to amplify the optical signals.
2. Background and Relevant Art
Optical communication systems have several advantages over other types of telecommunications networks. Optical fibers are typically made from insulative materials and are therefore less susceptible to interference from electromagnetic sources. Optical fibers also have higher bandwidth capability for both analog and digital forms of data. In addition, optical fibers are both smaller and lighter than metal cables.
As optical signals are transmitted through the optical fibers of a communication network, the optical signals gradually become weaker over distance. Thus, the optical signals need to be refreshed or strengthened before the signals become too weak to detect. Before the advent of optical amplifiers, regenerators were used to refresh or strengthen the weakened optical signals. Regenerators convert the optical signal to an electrical signal, clean the electric signal, and convert the electrical signal back to an optical signal for continued transmission in the optical communication network.
Optical amplifiers, on the other hand, are superior to regenerators for several reasons. Optical amplifiers are not as sensitive to bit rates and modulation formats as regenerators. Optical amplifiers can also be used with multiple wavelengths while regenerators are often specific to a particular wavelength.
Erbium-doped optical amplifiers have been used to amplify optical signals directly without requiring conversion of the optical signal to an electrical signal and back to an optical signal. These optical amplifiers are pumped optically by an external source of energy, such as 980 nm and 1480 nm semiconductor lasers, to excite electrons in an erbium-doped section of an optical fiber. As an optical signal passes through the erbium-doped fiber, the excited electrons emit photons having the same wavelengths as the incident optical signal, thereby amplifying the optical signal. Erbium-doped optical amplifiers are particularly useful in wavelength division multiplexing (WDM) optical networks, since they generally do not generate cross talk between the various wavelengths. One significant drawback of erbium-doped optical amplifiers is their cost. In particular, the semiconductor lasers are quite expensive, and can typically raise the cost per amplifier to tens of thousands of dollars.
Another, less expensive, type of optical amplifier is a semiconductor optical amplifier (SOA), which are pumped electrically as opposed to the optical pumping of erbium-doped optical amplifiers. At a basic level, a semiconductor optical amplifier is created by joining a p-type semiconductor material with an n-type semiconductor material to form an active region in the depletion region of the pn-junction when the semiconductor optical amplifier is forward biased. Optical signals are amplified by the stimulated emission of photons as the optical signal propagates through the active region of the semiconductor optical amplifier.
Instead of simply using a pn-junction as the basis of the semiconductor optical amplifier, another semiconductor material is formed at the pn-junction of the semiconductor materials. The new semiconductor layer typically has a higher refractive index than the adjacent p-type and n-type semiconductor regions. This is useful to help confine the light to the active region of the semiconductor optical amplifier.
One of the drawbacks of semiconductor optical amplifiers is a phenomenon referred to as cross talk, particularly when wavelength division multiplexing (WDM) is used. When an optical signal is input to a semiconductor optical amplifier, the carriers in the gain region of the semiconductor optical amplifier are reduced. More specifically, the carrier concentration is reduced through stimulated emission and/or spontaneous emission. When another optical signal is also incident to the semiconductor optical amplifier, the carrier concentration has already been reduced by the first optical signal and the second optical signal will experience less gain. It is possible for the second optical signal to be absorbed in certain instances. Thus, the optical signal affect each other in ways that are detrimental to the amplification process.
Cross talk occurs because the transition of electrons from the high energy state to the lower energy state occurs very fast in semiconductor optical amplifiers. This enables the gain of the semiconductor optical amplifier to respond according to fluctuations of the input signal. As a result, semiconductor optical amplifiers are not suited for the amplification of multiple optical signals of varying wavelengths because of the detrimental effect of cross talk. Thus, more expensive erbium-doped optical amplifiers are widely used in WDM optical networks even though their cost is significantly higher than that of SOAs.
These and other problems are overcome by the present invention which is directed to a semiconductor optical amplifier that eliminates cross talk between optical signals that are being amplified. The present invention also relates to methods of manufacturing semiconductor optical amplifiers that substantially eliminate cross talk between optical signals. Thus, SOAs constructed according to the invention can be used in WDM optical networks, and are significantly less expensive than the erbium-doped optical amplifiers that have been conventionally used in such optical networks.
The semiconductor optical amplifier includes a broad area laser with an active region that includes quantum wells. Typically, some of the quantum wells are compressively strained while other quantum wells are tensile strained. The semiconductor optical amplifier also includes a polarization adjusting layer whose thickness can be adjusted after manufacturing such that the transverse electric mode gain of the semiconductor optical amplifier substantially balances the transverse magnetic mode gain of the semiconductor optical amplifier. In effect, the polarization adjusting layer makes the semiconductor optical amplifier polarization independent.
When the broad area laser is lasing, the photon and carrier densities within the active region are at a threshold level and are substantially uniform across the active region. The facets at either end of the semiconductor optical amplifier are substantially perpendicular to the direction of the laser light, thereby reflecting the light and enabling the gain medium to operate as a laser. A ridge waveguide formed on the semiconductor optical amplifier structure guides optical signals incident in the active region through the semiconductor optical amplifier. The optical signals pass through the gain medium that is shared by the broad area laser, but at an angle that is displaced from the direction of the light generated by the laser. Because of the angular displacement of the optical signal, the optical signal is effectively not reflected internally in the gain medium and, therefore, is merely amplified instead of acquiring laser characteristics.
The incident optical signals experience gain without depleting the photon density or the carrier density. As a result, cross talk is substantially eliminated as the semiconductor optical amplifier shares the same gain medium as the broad area laser. In this manner, the semiconductor optical amplifiers of the invention can be used in a variety of optical networks, including WDM optical networks, replacing the more expensive erbium-doped optical amplifiers.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.